Zoho Books - Billing for the same project with 2 methods

Zoho Books - Billing for the same project with 2 methods

I have some projects that are a Fixed fee project, but after certain amount of hours, I need to invoice hourly for the same project.

When I tried to change the billing method to Per-User Hourly Rate, I got an error that there are invoices issued before and the billing method can't be changed.

The case is as follows:

We have a case that is a Lump sum for 5000USD (Invoiced as fixed fees) with cap hours of 30. After the consumption on the 30 hours we shall bill for extra hours consumed with (Hourly Rate).

That is for the same project and same client.


Please advise.